Mercury adsorbers have been used to remove heavy metals such as mercury from the natural gas to specifications, e.g., 0.1 μg/m3 (micrograms of Hg per normal cubic meter of gas), with the use of an adsorbent bed of carbon, zeolite, or supported metals known in the art. The adsorbents are rather expensive and require disposal as a hazardous waste, or shipped to a re-processing facility. When natural gas is to be liquefied using mercury-reactive alloys, such as aluminum, the mercury content of the gas must be reduced to or below 0.01 μg/m3.
When condensable hydrocarbons or water is present in the gas and with the use of commercially available adsorbents, condensed liquids may block the adsorption of the elemental mercury or cause the adsorbent to lose mechanical strength. The weakened adsorbent can crumble and may lead to plugging in the adsorber. In crude and gas production, the mercury-containing gas is often obtained from separators or from compressor-chillers. In both cases, the gas can be at or near its water and/or hydrocarbon dew point. To minimize problems from loss of the adsorbent, the gas is often heated to temperatures above its dew point. Alternatively, the gas can be chilled and the water and/or hydrocarbons condensed. The gas is then reheated prior to the mercury adsorption step. In both processes, expensive equipment is required. Also, the condensed water and hydrocarbon liquids from the second alternative can contain mercury and require additional treatment. It is recommended that hydrocarbon gases be heated to 28° C. above their hydrocarbon dew point to assure that no liquids condense.
Disposal of the spent adsorbents can be difficult in some locations with expensive retorting or disposal in hazardous waste landfills. Alternative disposal methods for the mercury are desired.
There is still a need for improved methods and systems to remove volatile mercury from fluids such as natural gas, vent gas or flue gas, allowing for the optimization of plant operations to minimize the equipment corrosion and mercury emission.
There is still a need for a process to remove mercury from gas and which is not affected by condensable hydrocarbons and water, to capture the mercury as either an aqueous waste that can be disposed by injection in a deep well, or as a small volume of mercury sulfide.